<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    var buildTree = function (preorder, inorder) {
      if (!preorder.length) {
        return null
      }
      const val = preorder.shift()
      const index = inorder.findIndex(item => item === val)
      const left = inorder.slice(0, index)
      const right = inorder.slice(index + 1)
      return {
        val,
        left: buildTree(preorder.slice(0, index), left),
        right: buildTree(preorder.slice(index), right)
      }
    };
    console.log(buildTree([3, 9, 20, 15, 7], [9, 3, 15, 20, 7]));
  </script>
</body>

</html>